How to calculate 1s and 2s compliment of binary number in C# and VB.Net

Please refer this code

I have given input in 4 bits form

HTML:

<form id="form1" runat="server">
<div>
    Enter 4 bit Binary number
    <asp:TextBox ID="txtBinaryNumber" runat="server" />
    <br />
    <asp:Button ID="Button1" runat="server" OnClick="GenerateComp" Text="Generate 1's and 2's Complements" />
    <br />
    1's Complement
    <asp:Label ID="lblOnesComplement" runat="server" />
    <br />
    2's Complement
    <asp:Label ID="lblTwosComplement" runat="server" />
</div>
</form>

VB:

Protected Sub GenerateComp(sender As Object, e As System.EventArgs) Handles Button1.Click
    Dim bits As String = Me.txtBinaryNumber.Text
    Dim onceComplement As String = String.Empty
    For i As Integer = 0 To bits.Length - 1
        onceComplement += If(bits(i) = "0"c, "1", "0")
    Next

    Dim twosComplement As String = String.Empty
    Dim carry As String = "0"
    Dim lastPosition As Integer = onceComplement.Length
    Dim ch As String = onceComplement.ElementAt(lastPosition - 1).ToString()
    If ch = "0" Then
        twosComplement = onceComplement.Substring(0, lastPosition - 1) + "1"
    Else
        For i As Integer = 0 To onceComplement.Length - 1
            ch = onceComplement.ElementAt(lastPosition - 1).ToString()
            If ch = "1" AndAlso carry = "1" Then
                twosComplement += "0"
                carry = "1"
            ElseIf ch = "0" AndAlso carry = "0" Then
                twosComplement += onceComplement.ElementAt(lastPosition - 1).ToString()
            ElseIf ch = "0" AndAlso carry = "1" Then
                twosComplement += "1"
                carry = "0"
            ElseIf ch = "1" AndAlso carry = "0" Then
                twosComplement += "1"
                carry = "1"
            End If

            lastPosition -= 1
        Next
    End If

    Me.lblOnesComplement.Text = onceComplement
    Me.lblTwosComplement.Text = twosComplement
End Sub

Using C#:

protected void GenerateComp(object sender, EventArgs e)
{
    string bits = this.txtBinaryNumber.Text.Trim();
    string onceComplement = string.Empty;
    for (int i = 0; i < bits.Length; i++)
    {
        onceComplement += bits[i] == '0' ? "1" : "0";
    }

    string twosComplement = string.Empty;
    string carry = "0";
    int lastPosition = onceComplement.Length;
    string ch = onceComplement.ElementAt(lastPosition - 1).ToString();
    if (ch == "0")
    {
        twosComplement = onceComplement.Substring(0, lastPosition - 1) + "1";
    }
    else
    {
        for (int i = 0; i < onceComplement.Length; i++)
        {
            ch = onceComplement.ElementAt(lastPosition - 1).ToString();
            if (ch == "1" && carry == "1")
            {
                twosComplement += "0";
                carry = "1";
            }
            else if (ch == "0" && carry == "0")
            {
                twosComplement += onceComplement.ElementAt(lastPosition - 1).ToString();
            }
            else if (ch == "0" && carry == "1")
            {
                twosComplement += "1";
                carry = "0";
            }
            else if (ch == "1" && carry == "0")
            {
                twosComplement += "1";
                carry = "1";
            }

            lastPosition--;
        }
    }
    this.lblOnesComplement.Text = onceComplement;
    this.lblTwosComplement.Text = twosComplement;
}

Image:

Thank You.